Tone-shift carrier



April 27, 1954 J. D. MoYNlHAN TONE-SHIFT CARRIER Filed June 29. 1950 John D. Moynhon. BY

ATTOR N EY Patented Apr. 27, 1954 UNITED STATES PAT-ENT OFFICE TONE-SHIFT `CARRIER John D. Moynihan, Bloomfield, N. J., assignor. tov

Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 29, 1950, Serial No'. 171,192

Claims. 1

My invention relates to an intelligence-transmitting means using two tones which are impressed upon a carrier-current wave, preferably by amplitude-modulation of the carrier, although the carrier might be frequency-modulated. It is a distinctive feature of my invention that one of the two tones which are used in the transmission of any bit of intelligence isV normally being transmitted, when there is nc intelligence to be transmitted, and is used, at the receiving end, as a restraining means or as alock-out means, for preventing any signal-response. When intelligence is-to be transmitted, the tone-frequency is changed or shifted from the normal tone-irequency to the other tone-frequency of the pair oi tones which are associated with that partcular piece of intelligence, and hence, at the receiving end, the restraint or response-prevention is removed, and the receiving apparatus is free to respond to the second tone, thus completing the transmission of the intelligence in question.

The word carrier is here used as indicative of any high-frequency currents which are used as carrier for transmitting a superimposed tonesigual having a much lower frequency than the carrier. rlhe carrier current may be either Ya radio-frequency current which is superimposed upon one of the line-conductors of a transmission-line, or an uitra-high-requency microwave current which is beamed to a remote terminal of a transmission system by means of a directional antenna located at the relaying station or line-terminal, or any other kind of carrier wave which is used in any kind of communication system.

Heretofo're, the use oi tone-signals has been considered for reiaying purposes, but the timedelays which are involved have been such as to make the direct keying of the tone-transmitter too slow forl manyre'laying purposes. It requires about live cycles oi the'tone-frequency, after keying-on a tone, to build up the magnitude of the tone to a usable value. After the turning off of the tone-transmitter, it requires about ten cycles of the tone-frequency for' the tone to die out. With my frequency-shifting system, it requires only about two cyclesV of the tone-frequency to shift from one tone-frequency to a distinguishably different tone-frequency.

An important advantage of my present system is to avoid the `possibility of incorrect operations or responses, as a result of interference caused by transients. Generally, a transient that will generate one tone-frequency, will generate a wide band of tone-frequencies in that vicinity, dueto the steep wave-iront of the transient, thus simultaneously producing both of the tones which are associated with any particular kind of intelligence which is to be transmitted. In using my invention, the restraint which isproduced by one of these tones is so strong, or so eiectively applied, that no response can be obtained 'when rior-current modulation, particularly in .it ose carrier-systems which are Subject to wide and sudden iuctuations of attenuation in the carrier'- current transmission, because, in my system, no attention needs to bepaidto either the amplitude of the modulation or signal, or the amplituderatio of the signal to; the unmodulated carrier wave. In my system, it is possible to selectively pick out the' tone er the pairoiltones toev 'h aA response is to be obtained, and then am said tones to a constant amplitude, wine be the `same regardless of the carrier-current attenuation, and then the response can be made to' the frequency'offthe toneqor to the frequency'- shift of the tone. i K v With the foregoing and other objectives in mind, my inventionl consists in the circuits, systems, combinations, apparatus, partsgand meth-- ods of design and use, hereinafter described and claimed', and illustrated in the accompanying drawing, wherein Figure l is a diagrammatic View of circuits and apparatus illustrating the receiving end of a tone-shitting carrier-currenttransfer-tripping relaying system using my invention;

of my invention; and

Fig. 3 is a block diagram of circuits and ap paratus illustrating complete six-tone carriercurrent system using my present' invention. ,.v f; i

Fig. l shows the tone-responsive'end of n Vexiemplary application of my two-tone carrier-current receiver, as applied to a transfer-tripping operation in which the intelligence which is to be received is information concerning the necessity for energizing the trip-coil TC of a line-sectionalizing circuit breaker, in response to fault-conditions which are detected at another terminal of the protected line-section. The apparatus shown in Fig. Iv includesa band-pass -lter F12', a limiter L 12,- a discriminator 1, amplifier-tubes AI and A2, and vrelays SG-I, SGZ, SG, and X2.

The lter' F12 is anysuita'ble type of band-pass filter which will permit thek passage of the two tone-frequencies f1 and fz which-'jointly cooperate in the transmission of the particular kind of intelligence' in question. 'Ilhis filter should have sharp cutoff points, so as to substantially prevent the transmission ofv any higher or lower frequencies through saidl lter. There are a number of! forms' of band-pass-lters which will per Fig. 2 is a tone-frequency diagram illustrative,

form such a service. I)The illustrated filter F12, in Fig. l, has two input-leads 'ict and lo. A iirst parallel-resonance device 8 is connected across said leads, followed by a series-resonance device E, which is in turn followed by a second parallelresonance device l0, connected in shunt across the supply-conductors, and finally a shunt-connected resistance Ro.

The output of the band-pass filter F12 is supplied to a limiter or limited-amplitude amplifier Llz, through a transformer Il. The purpose of the limiter L12 is to neutralize the effects of a variable strength or amplitude in the incoming tone-frequency wave, and to produce a tone of the same frequency, having a constant predetermined amplitude or intensity. There are a numher of limiters which are available for producing such a function. The particular one which is illustrated in Fig. l consists of an overloaded class-C amplifier-tube l2 in which the grid-circuit i3 is connected to one output-terminal of the transformer Il, and in which the cathodecircuit i4 is connected to a cathode-circuit resister l5, and thence to the other output-circuit lli of the transformer Il, and also to the negative battery-terminal The cathode-circuit resistor l5 is shunted by a capacitor C1. A second capacitor C2 is connected between the gridcircuit i3 and the cathode-circuit I4.

The plate-circuit I'I of the limiter-tube i2 serves as the output-circuit of the limiter L12, which is coupled to the discriminator 1 by means of a transformer i8. The essential feature of the discriminator 'l is that it shall have two direct-current output-circuits 2l and 22, the voltages of which shift in opposite directions with respect to each other, the direction of shift being dependent upon whether the one tone or the other is being received.

In the particular discriminator 1 which is shown in Fig. 1, there are two separately tuned disciiniinators, one of them having the tonefreduency f1 of tone i for its resonant-frequency fo, while the other discriminator has the other tone-frequency f2, corresponding to tone 2, for its resonant-frequency fn. The connections of these two discriminators are such that each produces a aero output-voltage, in its output-circuit 2| or 22, as the case may be, when it receives a pure tone of the same frequency as its resonance-freduency, but produces a unidirectional outputvoltage at any other frequency of tone-signal. l am not limited, however, to this particular type of discriminator, the only really essential thing being that I shall obtain a discrrninator-respense which discriminates between the two input-frequencies for which the discriminator is adjusted.

in the discriminator 1 of Fig. l, the two individual discriminators are alike, so that a description of one will suffice for both, with the understanding that the resonant-frequencies are different. shown, having a discriminator-transformer 23 in which the primary winding 24 is coupled to the mid-point 25 of the secondary winding 25 by a coupling-capacitor C3. The secondary terminals 2l and 28 are connected by a tuning-capacitor C4, which tunes the secondary circuit to the selected resonance-frequency fo which is the saine, say, as the shifted-tone frequency f2 of tone 2. The secondary terminal 21 is connected to the output-circuit 2| through a rectifier-tube Tl. The other secondary terminal 28 is connected to an output-circuit 29 through a second rectifier A very simple form of discriminator is 4 tube T2. The two output-circuits 2| and 29 are joined by means of a resistance Ri, the midpoint of which is connected to the mid-point 25 of the secondary winding 26.

The adjustment of this diseriniinator-circuit is such that the direct-current output-voltages across the two halves of the resistance Ri are equal and opposite to each other when the inputfrequency into the discriminator is the saine as the resonance-frequency fo to which this discriminator-circuit is tuned, so that no voltage appears in the output circuit 2l under these circumstances. When the other tone-frequency is fed into the discriminator-circuit in question, a predetermined positive voltage appe rs in the Output-circuit 2 l.

The other discriminator-circuit, shown in Fig. l, is similar to the discriminator-circuit just described, except that its secondary-winding terminals 2l and 22 are reversed, and its tuning capacitor C2i is of such size as to tune the secondary circuit to a resononce-frequency fo which is the same as the unshifted tone-frequency f1 of tone i, so that this second discriminator-circuit develops a zero voltage in its output-circuit 22 when tone l is received, while it develops a predetermined positive voltage in its outputcircuit 22 when tone 2 is received.

The operation of my apparatus, as thus far described, is illustrated in Fig. 2, wherein responses (current-strengths or voltages) are plotted against frequencies. Curve (a) of Fig. 2 shows the effect of the band-pass lter F12, which is tuned to a center-frequency fc, halfway between the two tone-frequencies f1 and f2 which are to be passed by the filter. This curve (a) shows how the filter passes the two desired tonefreduencies f1 and f2, but does not pass a substantial amount of current having either the next higher tone-frequency f3 or the next lower tonefreduency je.

fn Fig. 2, the second curve (b) shows the magnitude of the output-voltage of the first discriminator-circuit, as it appears in the output-circuit 2i, plotted as a function of the frequency. lit shows that this output-voltage is zero at the resonant frequency, which is f2 for this discriminator-circuit, while, for the other received tonefrequency f1, the output-voltage of this discriminator-circuit has its maximum positive value.

In Fig. 2, the third curve (c) shows the magnitude of the output-voltage of the second discriminator-circuit, as it appears in output-circuit 22, and shows that this output-voltage is nero when the frequency fr is being received, while it has its maximum positive value when the other frequency f2 is being received.

The first output-circuit 2l of the discriminator l, in Fig. i, is applied to the grid of an amplifier tube Ai, the plate-circuit of which contains the coil of a relay SGi which thus responds to the frequency f1 of tone I. In like manner, the second output-circuit 22 of the discriminator i is applied to the grid-circuit of an amplifier tube A2 which energizes the operating coil of a second relay SC-2, which thus responds to the frequency f2 of tone 2.

The first relay SG! has a bach-contact 3l, which is connected in series with a make-contact 32 of the second relay SGL', to energize the operating coil of a relay SG. The last-mentioned relay SG has a back-contact 33 and a make-contact 34. The back` SG contact 253 is in an energizing circuit for the operating coil of a slowvdropout relay X2, which is shown as having a acuer/i short circuited coil or slug Swhich delaysv its dropout action for a suitable time. The slow# dropout relay X2 has a back-contact :Sii which is connected in series with `the SG make `contact 34 in a tripping circuit which ener. isos-the breaker trip-coil TC in response :to theV SG re"L lay, before the slowerracting X2 relay can open its back-contact 3G.

In Fig. 3 I have indicated, by block diagram, an illustrative system in which my invention can be used or applied. In any tone-signal or fre: quency-signal system, where the transmission of intelligence is dependent upon the transmission or non-transmission of one or two selected fre.- quencies, whether audio-frequency tones, or supersonic tones or radio frequencies, it is prac.- tically always true that there will be other in,- formation-transmitting channels using other se,- lected tones or frequencies in the adjacent spectrum. This will be true, whether the selected tone or frequency is superimposed on a carrier wave, or transmitted by itself. There are also various kinds of intelligence which are to be transmitted, including printing telegraph sys'f tems, as well as the kind of intelligence-transmitting system which is associated withv the oper: ation of alternating-current power-transmitting systems for which my invention was primarily designed. By way of illustration, therefore, I

chosen to represent a system, using my in,- vention, which is' adapted to the needs of power-,- line operation, with the understanding, however, that my invention is not limited to this particular held of application.

In Fig. 3, therefore, I have shown a three-phase transmission-line section 4i which is connected to a station-bus 42 through a circuit breaker CB having a trip coil TC. Various types of intelligence need to be transmitted from one station to another, during the successful operation of the line lli, Such information can be transmitted through various kinds of communicating channels, such as a radio-frequency carrier-wave which is superimposed upon the power-line 4i, pilot wires owned by the power company, telephone or telegraph wires or channels leased by the power company, or directional radio or microwaves which are not guided by the power-line conductors.

I have chosen to illustrate, in Fig. 3, a commu-f nicating channel consisting of a carrier wave which is produced by a carrier transmitter 43, and which is coupled, by a transformerV 44, to one of the phase-conductors ofthe power-line 4i, through a tuned circuit 4S and coupling capacitors 4l'. At the same station, there will generally be carrier-receiving apparatus, which may be connected onto a tapped point 48 in the couplingtransformer Lift, through a tuned receivencircuit 139. As the same equipment is, or may be, used at both ends of the protected line-section 4i, I have illustrated only one end or line-terminal, with the understanding that the other terminals may be duplicates of the illustrated terminal, except, possibly for the tone-frequencies which are used.

In Fig. 3, I have illustrated, by block diagrams, three typical kinds of information which have to be transmitted, at times, from one end of the protected line-section to the other, during the successful operation of the line, and for protecting the line against faults. Thus the block 5i indicates a transfer-tripping control-apparatus, which determines, from information availableat the relaying station in question, that a fault exists Within the protected line-section di, and that it is necessary to transfer or transmit this information to the remote terminal .of the protected lines section, in order .totrip the circuitbreaker which is located at said remote terminal.

I have also illustrated afault-responsive car.- rierl-control.apparatus52, in which information which is received, by carrier, from the remote terminal of the vprotected line-section is com.- pared with information obtainablcat the relaying station, for the purpose of determining whether any given faultion theA transmissio'nesystem is located within the confines. of theprotected linee section 4|.

I have also illustrated a telemetering control,- apparatus 53, whereby information concerning the generating conditions and the loadedistribu: tion at the relaying station can be transmitted to a load-dispatcher at some other point in the transmission system, or whereby the dispatcher at v,the relayingr station can operate automatic loadfcontrol equipment at some remote station.

As rvarious means are known, for these `three types of .carrierecontrolling apparatus, they ,are` indicated only by blockfdiagram, as being under the control of a setof line-,current transformers 54 and potential transformers V55.

Two tones or frequencies are assigned to each carrier.,-,.controll apparatus, thus making Ya spec? trum of six tones or irequencieswhih are in; volved in the transmission of thi-ee kinds of i` telligence, as in the particular, communication,- system which is shown inFig. Thus, the transf fer-tripping controlrapparatus 5i controlsan Qs,- cillator O12 which iscontinuously oscillating to` produce either tone i or tone 2 orl ther ,the frequency 11, or the frequency f2, shifting vfro one tone to the other, under the control of transfertripping controleapparatus 5 l. lhe output of this oscillator O12 is shown as being next passed through a limiter or Vwave-Shaper Lig, and thence on into the modu1ator-part El of the car,- rier-current transmitter43. The carrier-current modulation may be either amcltudemodulatios or frequencyfmodulation. At present, amplitudemoduiaimnis ,the preferred sind, and, l 'have se indicated, in the diagrammatic represent-ation of the modulatQr Y In like manner, thev faultfrespcnsive, .Sarriericonti-,ol apparatus 52 Cent-r0.1@ a ,tene-,estilista O34, which produces either tone 3 or tone li, hav.- ing either the frequency ,f3 or fi, according to the controlwhich it receives; and this oscillator 0.3i is. fconnacted` to, the amplitudefmudclatvr 5l through its own limiter L34. In the saine way,v the telemeteringfcontrol apparatus 53 controls its oscillator O56, producing tone 5 or tone E, havingvthefrequerwy for f6, transmitting the Samey through its limiter L55, to the modulator 57. l The carrier-receiving equipment, in Fig. 3, is` illustrated as including, rst, acarrier-frequency limiter 59, which is coupled to the tuned receiver# circuit 4,9, and which limits the amplitude of the carrierwave which is passedv on to the sensitive detector 60, so that the detector will not be. damaged by the high-amplitude carrier-wave which is received from the local transmitter 4 3.

The detector 6i! has an output which contains all of the tone-frequencies which are included` in the received carrier wave. This detectoroutput is passed on to three band-pass filters F12, lsiA and Faaeach of which is designed to, pass only the two adjacent tones, in the tonespectrum, to which its apparatus is intended to be selectively responsive. While I have sed the saine numbers to designate the tones in the receiving apparatus, `as in designating the tones in the corresponding transmittercontrolling apparatus, it is to be understood that the receiving apparatus is designed to respond to certain tones which are transmitted from the other line-terminal, which is not illustrated, While the transmitter-controlling tones are intended to actuate the receiving apparatus at the remote lineterminal. In most cases, a different set of tones is used in the transmitter-controlling apparatus at each station or line-terminal, in order4 that separate information may be vtransmitted both Ways, without interference. This is not necessarily so, however. At any rate, it should be understood that the tones which are referred to, in describing the receiving apparatus, are the tones which are used in controlling the transmitter at the other end of the protected linesection 4l.

The outputs of the three band-pass filters F12, F34 and Fss are iirst passed through their respective limiters' or amplitude-limited ampliers L12, La/l andI Ls, which bring up the respective pairs of tones, if they are present in suicient Volume, to a substantially Xed amplitude, regardless of fluctuations in attenuation during the course of transmission.

The constant-amplitude tones which are produced by the respective limiters L12, Lsl and Lsa are next passed on to suitable discriminators which have a separate output-circuit for producing direct-current voltages which are selectively varied in response to each of the received tones, respectively. Any sort of discriminatorapparatus or arrangement, which Will accomplish such a function, may be used, so far as the workings of the system as a Whole are concerned. I have indicated separate discriminators D1. D2, Da, D4, D and De, one for each tone, where the subscript designates the number of the tone to which the resonant-frequency fo of the discriminator is tuned, but I intend such illustration to be indicative, broadly, of the use of any sort of discriminator-means for distinguishing between the frequencies of the pair of tones which are used in each of the intelligence-transmitting channels.

The limiter L'iz is shown as feeding the discriminators D1 and D2, while the limiter' Ln feeds the discriminators D3 and D4, and the limiter Ls feeds the discriminators D5 and De. Thus, each of the six discriminators is impressed with input-currents having either one or both of two tone-frequencies. Each discriminator produces no (significant) voltage in response to an input-tone having the same frequency as its resonant-frequency fo. Hence each discriminator produces a (signicant) direct-current outputvoltage only in response to the other one of the two input-frequencies with which it is supplied. Thus, the discriminator D1 produces an outputvoltage E2 which is responsive to the received tone 2, while the discriminator D2 produces an output voltage El which is responsive only to the received tone I. In like manner, the discriminators D3, D4, D5 and De produce the output voltages E4, E3, Es and E5, each of which is responsive only to the received tone designated by its subscript.

There are thus produced two discriminatorvoltages for each of the types of information which are to be received by the receiving equipment illustrated in Fig. 3. In each case, it is assumed that the corresponding transmittercontrolling apparatus at the remote station is normally continuously producing the lowernumbered of the two tones which are assigned 8 to the transmission of that particular class of intelligence, and it is assumed that the intelligence is transmitted by sluiting the tone from the lower numbered tone to the higher numbered tone, of that pair, and back again, in accordance with the transmitter-controlling apparatus.

In accordance with my present invention, the rst, or lower-numbered, tone o each pair is used, in the receiving apparatus, as a signalblocking, signal-restraining or signal-preventing means, While the other tone of that pair is used as the signal-operating means, the restraint being so strong or so eiiective that a signalling operation is not obtained il both oi the tones should be received or detected simultaneously. It is obvious that various means could be used Whereby this sort of receiver-operation could be obtained, whereby an operation is produced only if a second tone is received in the absence of a first tone. It is intended. therefore, that the illustration in Fig. 3 be regarded as a diagrammatic illustration representative of any receiver- .cans for eflecting the broad objectives just eX- plained.

In Fig. 3, the first pair or" discriminators D1 and D2 are used to control a transfer-tripping relaying-apparatus 5l. Thus, the tone 2 discriminator-voltage is fed into the transfertripping relaying-apparatus Si through the back contact Xi of a relay XI, t. e operating coil of which is energized by the discriminator-voltage E1, which is produced when tone i is being transmitted.

The second pair of discriminators Da and D4 are similarly used to control a carrier-controlled relaying-apparatus 62. The discriminator-voltage E4, which is responsive to the second tone of that pair, is supplied to said carrier-controlled relaying-apparatus 62, through the backcontact X3 of a relay X3 which is energized by the discriminator-voltage E3 which is responsive to the tone 3 of the pair. The carrier-controlled relaying-apparatus 32 is shown as having a second input or control-line which is energized from the output of the local -fault-responsive oarrier-controlling apparatus 52, so that the carrier-controlled relaying-apparatus t2 can comparatively respond to both local line-conditions and remote-end line-conditions at the other terminal of the protected line-section fil.

The third pair of discriminators D5 and De, in Fig. 3, are shown, by way oi illustration, as controlling a differential type of telemetering receiver-relay TM, which has an operating coil O6 which is energized from the discriminator-voltage Es in response to the second tone (i oi that pair, and a restraining coil Rd which is energized from the discriminator-voltage Et which is responsive to the iirst tone 5 of that pair. It is understood, of course, that the differential type of relaying apparatus, which is shown at TM, could be replaced by the type of control which is used for the relaying equipments tu' and 52, or vice versa.

The transfer-tripping relaying-apparatus E! and the carrier-controlled relaying-apparatus 62 are both used to control the energization of the trip coil TC of the local line-sectionalizing breaker CB, as shown by the diagrammatically indicated trip-circuit iid. The telemetering relay TM is used to control telemetering apparatus (not shown), which may be conventional.

An important advantage of the shifting-tone intelligence-tranmitting equipment, as illustrated in Fig. 3, is that the same number of tones are ,29 alwaysbeing transmitted by the communicationsystem, whether that transmission be by Way of separate tone-frequencies-Which are being modulated onto a carrier wave, or Whether said transmission he by way of separate independent tonefrequencies or radio-frequencies which are transmitted hy themselves, without being superimposed upon a higher-frequency carrier-wave. Each tone, which is used, out of an available sequence or spectrum of tones or frequencies necessarily causes some interference with other tones of the same spectrum or series. ThisV is inevitable, notwithstanding the fact that various precautions are used, such as by Way of the limiters or wave-Shapers L12, L34 and Les the transmission-control. Such interference will tend to cause false tone-transmission, and' hence false tone-reception; and that interference may load a receiver until said receiver cannot discriminate between tone and 11o-tone conditions.

The intelligence-communicating system, shown inlig. 3, eliminates this tone-interference trouble by leaving each tone-transmitter O12, Osiand O56 connected, and operating, at all times. Each tone-transmitter thus loads the other 'tonetransmitters, or'produces side-band tones having the same frequencies as the other tone-transmitters, but this loading now becomes constant, because the tones are always there, and hence such a loading becomes negligible, because the receiving apparatus can be adjusted to discriminate against such constant interference. This system is `nroadly covered in a copendingapplication of B. Lenehan, Serial No. 171,141, led June 29, 1950.

In the intelligence-communicating system, as used in my present invention, and as shown in Fig. 3, a proper discrimination between tone and no-one conditions is also achieved through the use of a type of receiving apparatus which responds to one tone, and is restrained or locked out by another tone. This sort of safeguard is particularly advantageous in protecting the apparatus against the sort of interference Which comes from transients of various kinds. Generally, a transient which will generate one frequency will generate a wide band of frequencies in that vicinity, thus producing both tones of each pair of tones which are dierentially used in my tone-receiving apparatus, the apparatus being so designed that it Will not respond, if both tones are received at the same time.

While I have illustrated my invention, and indicate-d its principles of design, operation and application, with several examples, wish it to be understood that my invention is susceptible of considerable modiiication, by Way of substituting equivalents, or omitting or adding various parts and renements, without departing from the essential spirit of the invention. I desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language.'

I claim as my invention:

l. A frequency-shifting communication-system comprising a continuously transmitting transmitter normally transmitting a first frequency, signalling controller-means acting on the transmitter for shifting its frequency to a distinguishably different frequency, signal-receiving means for selectively responding to the shifted frequency of said transmitter, and signal-blocking means selectively responsive to the unshifted frequency of the transmitter fory preventing a signal-response.

2. The invention as defined in claim 1, characterized by said signal-receiving means including a discriminator having its resonance-frequency substantially equal to the unshifted frequency of the transmitter, and said signal-blocking means including a discriminator having its resonancefrequency substantially equal to the shifted frequency of the transmitter.

n 3. A frequency-shifting tone-signal communication-system comprising a continuously transmitting carrier-current transmitter, a shiftablefrequency tone-frequency modulator for continuously superimposing a tone-frequency modulation on the carrier current, said tone-frequency being much lower than the carrier frequency, signalling controller-means acting on the modulator ,for shifting itsA frequency to a distinguishably different tone-frequency, and receiving appara-tus, including means for receiving said modulated carrier current, signal-responsive means for selectively responding to the shifted tonefrequency of said received modulated carrier current, and signal-blocking means selectively responsive to theunshifted tone-frequency of said received modulated carrier current for preventing a' signal-response.

4. The invention as defined in claim S, characterized by said signal-responsive means including a discriminator having its resonance-frequency substantially equal to the unshiited tone-frequency of the received modulated carrier current, and said signal-blocking means including a discriminator having its resonance-frequency substantially equal to the shifted tone-frequency of the received modulated carrier current.

5. A multi-channel frequency-shifting tonesignal communication-system comprising a continuously transmitting carrier-current transmitter, a plurality of shiftable-frequency tone-frequency modulators, each continuously superimposing a distinctive tone-frequency modulation on the carrier current, said tone-frequencies being much lower than the carrier frequency, a separate signalling controller-means acting upon each modulator for shifting its frequency to a distinguishably different tone-frequency, and receiving apparatus, including means for receiving said modulated carrier current, a plurality of signal-responsive means, each selectively responding to a particular shifted tone-frequency of said received modulated carrier current, and a signal-blocking means associated with each signal-responsive means for preventing a signal response, each signal-blocking means being selectively responsive to the corresponding unshifted tone-frequency.

6. The invention as defined in claim 5, characterized by each signal-responsive means including a discriminator having its resonance-frequency substantially equal to the corresponding unshifted tone-frequency, and each signal-blockingmcans including a discriminator having its resonance-frequency substantially equal to the shifted tone-frequency tov which the associated signal-responsive means is selectively responsive.

7. A frequency shifting communication system comprising a continuously transmitting transmitter normally transmitting a first frequency, signalling controller-means acting on the 'transmitter for shifting its frequency to a distinguishably different frequency, receiver-means for selectively receiving a frequency-band including said shiited and unshifted frequencies as substantial limiting-frequencies of the received band, and signal-receiving means for selectively 11 responding to the shifted frequency of said transmitter, said signal-receiving means including a discriminator having its resonancefrequency substantially equalto the unshifted frequency of the transmitter.

8, A frequency shifting tone signal communication-system comprising a continuously transmitting carrier-current transmitter, a shiftable-frequency tone-frequency modulator' for continuously superimposing a tone-frequency modulation on 'the carrier current, said tonefrequency being much lower than the carrier frequency, signalling controller-means acting on the modulator for yshifting its frequency to a distinguishably different tone-frequency, and receiving apparatus, including means for receiving said modulated carrier current, band-pass means for selectively receiving a frequency-band including said shifted and unshifted tone-frequencies as substantial limiting-frequencies of the received band, and signal-responsive means for selectively responding to the shifted tone-frequency of said received modulated carrier current, said signal-responsive means including a discriminator substantially equal to the unsliifted tone-frequency of the received modulated carrier current. 9. A multi-channel frequency-shifting tone signal communication-system comprising a continuously transmitting carrier-current transmitter, a plurality of shiftable-frequency tonefrequency modulators, each continuously super imposing a distinctive tone-frequency modulation on the carrier current, said tone-frequencies being muchlower than the carrier frequency, a

having its resonance-frequency separate signalling controller-means acting upon each modulator for shifting its frequency to a distinguishably different tone-frequency, and receiving apparatus, including means for receiving said modulated carrier current, a plurality of signal-responsive means, each including a bandpass means for selectively receiving a frequencyband including, as substantial limiting-frequencles of said band, the shifted and unshifted tone-frequencies of one of said modulators, and a plurality of signal-receiving means, each selectively responsive to the shifted tone-frequency of its band, each signal-receiving means including a discriminator having its resonance-frequency substantially equal to the corresponding unshifted tone-frequency.

10. In a relay system, a device to be controlled responsive to an intelligence signal, actuating means associated with said device, means comprising frequency yshift apparatus for applying a blocking signal to said actuating means at all times when an intelligence signal is absent and for removing said blocking signal during al1 times when an intelligence signal is present.

References Cited in the file of this patent UNITED STATES PATENTS Boughtwood Oct. 30, 1951 

